In recent years there has been an exponential expansion in the Internet and in the number of people who want to connect to the Internet. Businesses have found the Internet a cheap and efficient way of communicating information to their customers, to their suppliers, and even among their own workforce. Employees exposed to the Internet at work have gone in search of tools to connect the personal computers they have at home to the Internet so that they can have access to the vast resources they have become accustomed to at work.
The modem has filled this need for the past twenty years, but in the past ten years it has seen unprecedented advances in technology. With the advent of the World Wide Web associated with the Internet, engineers have consistently needed to push larger and larger amounts of data through a pipeline that has not really grown. In the past few years, with demand growing for “real-time” networks, designers have started to develop alternatives to the traditional modem after deciding that traditional modems most likely had a top speed around 56 Kbps. These include digital subscriber line (DSL) modems, integrated services digital network (ISDN), and cable modems.
DSL modems in particular have received a lot of attention recently. DSL modems operate at higher data rates through a combination of higher frequency transmission and by using mapping techniques to map a series of bits onto a single symbol. These techniques typically require that both the transmitter and receiver are in sync with each other. When the systems are not in sync, either or both of the receivers are looking at an incorrect portion of the signal. In such a situation, the systems are likely to see an incorrect phase angle or an incorrect magnitude, and the data ends up being misinterpreted.
In the past, synchronization has been done either through the use of a preamble, the use of an off frequency pilot tone or analysis of band edge signals. Using the preamble method, a known set of data is transmitted at the beginning of each transmission, and the receiver looks for this set of data and determines the characteristics of the transmission. The pilot tone method on the other hand, transmits a constant pattern of data (pilot tone) offset from the carrier frequency, thus allowing the receiver to derive the timing information from the pilot tone even in the absence of modulated data. The band edge method filters the signal at each edge of the modulated bandwidth then performs non-linear operations to measure the bandwidth or symbol rate. Each of these systems has certain disadvantages.